Injection molding assembly

ABSTRACT

Cellular organic polymer articles are made by an injection molding method in which a foamable thermoplastic melt is injected from an extruding device through a nozzle and sprue bush into a mold, the sprue bush being thermally insulated from the injection nozzle and the nozzle being closed at a position near its outlet after the filling step and maintained in injection position throughout the molding cycle.

United States Patent Czerski [541 INJECTION MOLDING ASSEMBLY [72]inventor: Jan Czerski, Crewe, England [73] Assignee: Shell Oil Company,New York, NY.

[22] Filed: April 8, 1969 [211 App]. No.: 814,365

[30] Foreign Application Priority Data April 11, 1968 Great Britain..l7,52l/68 [52] US. Cl. ..425/247, 164/303, 425/449 [51] Int. Cl...B29I 1/022 [58] FieldofSearch..... .l8/30WD,30NM,3ONA, 18/30 NB,30 NZ,30 RM; 164/303, 312, 316

[56] References Cited UNITED STATES PATENTS 2,698,464 1/1955 Wilson18/30 WD X 2,928,125 3/1960 Smucker et a1 ..l8/30 NBX 1 Oct. 17,1972

3,169,199 11/1963 Hardy ..18/3O RM 3,134,141 5/1964 Hardy ..18/30 RM3,504,733 4/1970 Smith ..18/30 WD FOREIGN PATENTS OR APPLlCATlONS1,399,530 4/1965 France ..l8/30 NH Primary Examiner-J. SpencerOverholser Assistant Examiner-Michael 0. Sutton Attorney.loseph W. Brownand Martin S. Baer [57] ABSTRACT Cellular organic polymer articles aremade by an injection molding method in which a foamable thermoplasticmelt is injected from an extruding device through a nozzle and spruebush into a mold, the sprue bush being thermally insulated from theinjection nozzle and the nozzle being closed at a position near itsoutlet after the filling step and maintained in injection positionthroughout the molding cycle.

1 Claim, 2 Drawing Figures PATENTEUIJCT n ma 3598.849

INVENTOR:

JAN CZERSKI BY: a

HIS ATTORNEY INJECTION MOLDING ASSEMBLY The present invention isconcerned with the manufacture of cellular articles from thermoplasticcompositions by injection molding, with the resulting cellular articlesand with machinery suitable for use in the manufacture thereof. The term"thermoplastic composition" is used herein to include compositions oforganic polymers which have thermosetting properties, for example, epoxyresins, but which are thermoplastic under the conditions of, or at thestage of, injection molding, as well as compositions of thermoplasticpolymers, such as polyolefins.

Such cellular articles are formed from a melt of a foamablethermoplastic composition. Such a foamable thermoplastic compositioncontains a foaming agent which, under appropriate conditions, willgenerate a cellular structure within said thermoplastic composition. Thefoaming agent may be a volatile, normallyliquid substance, for example,technicai pentane. Foaming agents of this kind are known in the art formaking expanded, i.e., cellular or foamed, polystyrene. Preferably, thefoaming agent is of the chemical type, i.e., a substance or mixturethereof which, for example, under elevated temperature conditions willform a gas, for example, carbon dioxide or nitrogen, capable of formingthe required cellular structure in a thermoplastic material. Foamingagents of this kind are known in the art for making, for example,cellular polyolefins and polystyrene. Whatever the kind of foaming agentused, the foaming depends upon the generation or release within themolten mass of a large number of discrete bubbles of gas or vapor which,as the pressure on the mass is decreased, expand to form the cells ofthe ultimate cellular article.

it has been found possible to manufacture cellular articles fromfoamable thermoplastic compositions by injection molding. The knownmethod and apparatus for injection molding of foamable thermoplasticcompositions is described in US. Pat. No. 3,268,636 to Angeli, Jr. Inthe apparatus illustrated in said patent, foamable mixture is extrudedinto an accumulator chamber through a three-way valve and the mold ischarged from the accumulator, through the same valve, in a quick surgeof expanding polymer composition.

The present invention is directed to an improved method and apparatusfor injection molding foamed thermoplastic compositions, andparticularly to an improved arrangement of connecting the source offoamable melt to the mold. The invention is based on the finding that,in charging a mold with a hot, foamable thermoplastic melt, improvedresults are obtained if, after the mold has been charged, the injectionnozzle is closed at a point as near as is practical to the nozzle outletand the nozzle is held permanently in the injection position. It wasfound that if the nozzle is retracted after injection, in accordancewith conventional injection molding practice, the foamable compositionremaining in the sprue tends to expand in the direction of the nozzleand to unite with the molten material in the nozzle tip; solidificationthereof makes it difficult or even impossible to remove the moldedcellular article from the mold in the normal way. However, it was alsofound that maintenance of the nozzle permanently in the injectionposition poses difficulties in cooling the sprue area and the adjacentpart of the mold, since the heat is transferred from the injectionnozzle to the sprue bush and mold.

According to one aspect of the present invention, a method ofmanufacturing a cellular article by injection molding comprises charginga mold with a melt of a foamable thermoplastic composition through asprue bush which is thermally insulated from the injection nozzle of theinjection molding machine, closing said nozzle at its outlet end whilemaintaining said nozzle in its injection position throughout the moldingcycle; and removing the molded cellular article. The present inventionalso includes the resulting cellular article.

Preferably, said thermal insulation is provided by an insulatingelement, suitably an apertured layer or disc of thermally insulatingmaterial disposed within a recessed portion of the sprue bush, and acorrespondingly apertured metal retaining element between saidinsulating material and the nozzle outlet, the outer surface of saidretaining element being recessed for the reception of the outlet end ofthe nozzle. Said insulating element can be formed in situ from a layerof thermally insulating material which is pressed into a recessedportion of the sprue bush and apertured for the passage of foamable meltby movement of the nozzle into its injection position followed by theinjection of melt from said nozzle.

If desired, the sprue bush and mold can be cooled in conventional mannerby the circulation of. a heat transfer liquid through appropriatecooling channels therein, but it is a particular advantage of thepresent invention that the above-defined method of manufacturingcellular articles can be carried out using molds of simple constructionin which special provision for cooling the sprue bush area is notneeded. For example, it has been found that the injection pressuresrequired when injection molding cellular polyolefin articles are suchthat relatively cheap molds made, for example, from cast aluminum, canbe used. It is, of course, advantageous in such case to be able to avoidincreasing the mold cost by dispensing with special means for effectingcooling of the sprue area after injection of the charge of foamablethermoplastic composition. Such molds can be cast in two or morecomponents, usually in two mating halves, one of which is provided withan integral pillar or collar portion which constitutes the sprue bush.Alternatively, a separate sprue bush can be used and removably securedto the appropriate mold component prior to mounting the mold in theinjection molding machine. In such case, a common sprue bush can beinterchanged between different cast molds, and in this way some capitalcost advantage in respect of the molds can be obtained; such a spruebush may have provision for cooling, if desired, but this is notessential. Advantageously, however, the sprue bush forms an integralpart of one of the cast mold components for simplicity in setting up theinjection molding machine.

The thermal insulation provided between the sprue bush and the injectionnozzle is such that, apart from the necessary feed passage between thenozzle outlet and the sprue channel, which feed passage forms in effecta short rearward extension of the sprue channel, there is no deadvolume" in which foamable melt can accumulate and cause trouble byfoaming when pressure is removed and the molding ejected. For thisreason, the use of an air space to reduce heat transfer,

for example, by arranging for the nozzle to enter a cavity in the spruebush which is of greater diameter than the nozzle and extends inwardlyof the sprue bush beyond the injection position of the nozzle, isunsuitable because in operation the space between the nozzle and thecavity wall becomes filled with foamable melt. ln carrying out thepresent invention, the required thermal insulation convenientlycomprises a layer or disc of thermally insulating material disposedwholly or partly within a recessed portion of the sprue bush surroundingthe sprue channel in the surface of the sprue bush which would normallybe contacted by the injection nozzle, the layer of insulating materialbeing apertured for the passage of foamable melt therethrough. Saidlayer can be formed, for example, at the commencement of an injectionmolding run by interposing a thin sheet of insulating material, forexample, polytetrafluorethylene or a resin-bound asbestos composition,between the nozzle and the dished sprue bush so that forward movement ofthe injection nozzle into its operative position followed by meltinjection forms the required annulus in situ; however, the insulation ispreferably preformed and inserted into the sprue bush prior to or duringsetting up of the machine.

The present invention can be applied to the molding of a wide variety ofcellular articles, for example, containers such as open-topped boxes andcases, for which lids may also be molded in a separate operation, andvessels such as bowls.

Suitable thermoplastic polymers for use in this invention include, forexample, olefin polymers and copolymers such as polyethylene,polypropylene and ethylene/propylene copolymers; polystyrene and vinylresins, including acrylonitrile/butadiene/styrene (ABS) polymers; thenylons and polycarbonates. The invention can also be employed in themolding of hardenable or thermosetting resin compositions which arethermoplastic under the conditions of, or at the stage of, injectionmolding, for example, epoxy resins. The term thermoplastic compositionis used herein to include resins or polymers as such or containingadditives, for example, stabilizers, antioxidants and coloring matter,as well as compositions comprising significant amounts of othercomponents, for example, fillers.

The foamable thermoplastic composition may comprise as the expanding" orfoaming" agent any known or suitable thermally or chemicallydecomposable or volatilizable foaming agents. Decomposable foamingagents include organic azo and nitroso compound, e.g., anazodicarbonamide which decomposes on heating to yield nitrogen orcarbonates or bicarbonates which decompose to yield carbon dioxide.Volatilizable foaming agents include volatile, normallyliquid organicsubstances such as hydrocarbons and halogenated hydrocarbons, e.g.,technical pentane. A foaming agent which yields nitrogen or carbondioxide on heating is preferred from the fire hazard point of view. Aso-called foam nucleating agent, which may be a finely-divided inertsubstance, for example, calcium silicate, may also be present, ifdesired, to aid formation of the desired foam structure.

The foaming agent can be mixed with a thermoplastic composition at anyconvenient and suitable stage, for example, within the screwplasticizing section of a suitably modified screw pre-plasticizinginjection molding machine in the case of a foaming agent such aspentane, or by homogeneously mixing it with the polymer prior to feedingit to the machine in the case of a foaming agent which decomposes toyield a gas on heating.

Usually a thermoplastic composition containing the required foamingagent, and optionally a nucleating agent, will be supplied as such tothe molder.

The kind of injection molding machinery which can be used in carryingout the present invention can be conventional to the extent thatmachines are available commercially which have a large shot capacity andare adapted to permit rapid injection of the charge into the mold, haveprovision for closing the injection nozzle at its outlet end at adesired stage in the molding cycle, and have means for retracting thenozzle, which means can be adjusted, in accordance with the presentinvention, to maintain the nozzle in the injection position throughoutthe molding cycle. Preferably, the injection nozzle is closable at aposition as near as possi ble to the nozzle outlet. If desired, one canuse an injection molding machine having a reservoir into which foamablethermoplastic composition can be fed and maintained under pressuresufficient to prevent foaming thereof until enough material toconstitute the charge has been built up in the reservoir; the charge isthen transferred rapidly from the reservoir to the mold. Suitableinjection molding machines may be designed on the basis of therequirements referred to herein, or existing types of machine, wherebasically suitable, can be modified to meet these requirements,whichever is the more convenient. In either case, provision is made forthermal insulation between the nozzle and the sprue bush in the mannerindicated herein.

The present invention also includes a machine suitable for use in themanufacture of cellular plastics articles from a melt of a foamablethermoplastic material by injection molding, which comprises aninjection molding machine having a valved injection nozzle adapted toremain in its injection position with respect to the sprue bush of amold during the whole of the molding cycle, said nozzle being thermallyinsulated from said sprue bush by an insulating element between theoutlet end of the nozzle and the sprue bush, which element is aperturedfor the passage of foamable melt therethrough.

Said sprue bush can be a separate sprue bush or it can be an integralpart of a mold component; in either case the required thermal insulationcan be provided by an insulating element of the kind previouslydescribed.

Advantageously, said injection nozzle is provided with heating means formaintaining the nozzle at the required operating temperature.

The present invention also includes a mold assembly for use in a machineas just defined, which comprises a mold part fitted with or comprising asprue bush having a recessed portion surrounding the outlet of the spruechannel on the injection side of the sprue bush, said recessed portionhousing an apertured layer or the like of thermally insulating materialand a correspondingly apertured metal retaining element the outersurface of which is recessed for the reception of the outlet end of aninjection nozzle.

The present invention will be further described with reference to theaccompanying drawings in which:

FIG. 1 is a sectional side view of part of a mold with its associatedsprue bush, and

FlG. 2 is a plan view of the same part.

Referring to FIG. 1, a mold back plate I carries a sprue bush 2 havingthe usual axial sprue channel 3 to the mold cavity (not shown). Thesprue bush 2, which is fixed to back plate 1 by a setting ring 2, isrecessed at 4 for the reception of an insulating element in the form ofan apertured insulating disc 5 which is held in recess 4 by a retainingelement in the form of a correspondingly apertured steel insert 6secured to sprue bush 2 by screws 7. lnsert 6, which is of square shapeto minimize its peripheral contact with sprue bush 2, is dished at 8 toa radius matching that of the outlet end of the injection nozzle of theinjection molding machine. The nozzle, which is shown in broken lines at9, is maintained during the whole of the molding cycle with its outletend in forced engagement with dished portion 8 of insert 6 in order toprevent leakage of foamable melt; heat transfer from nozzle 9 to spruebush 2 is virtually eliminated by means of disc 5 and the use of asquare insert 6 which contacts the sprue bush 2 only at its corners 6'.It will be appreciated, however, that other arrangements are possible.For example, a circular insert fitting with clearance within recess 4could be employed to retain disc 5, or the arrangement described in thefollowing example could be used, particularly for short runs or where amold must be adapted as cheaply as possible for operation in accordancewith the present invention.

In carrying out an injection molding operation in accordance with thepresent invention, the melt of foamable thermoplastic composition ismaintained under pressure conditions adequate to prevent undesirablefoaming thereof until the charge is injected into the mold. The mold isvented to the extent that the pressure on the charge therein is suchthat it can foam up to form a cellular molding. The injection pressureand the gate area, i.e., the cross-section of sprue channel 3, areselected such that rapid transfer of the charge into the mold takesplace, but the pressure in the mold itself does not attain high values,thereby permitting the use of the cheaper cast molds referred topreviously. The optimum conditions of operation within the generalconditions of operation defined herein, for example, the nozzletemperature, the melt temperature profile within the extruder and theinjection pressure may be determined by trial to suit the particularpolymer and foaming agent employed, and the nature of the moldedcellular article to be made. Once the general conditions of operationdefined herein have been satisfied, the derivation of such optimumconditions to suit the polymer used and the article made is conventionalpractice in the injection molding art.

The present invention is illustrated by the following example:

EXAMPLE A cellular bowl-shaped article was molded from a foamablepolyethylenic composition on a reciprocating type, single screwinjection molding machine. The machine has provision for screwpre-plasticizing the polymer feed and, by appropriate forward movementof the screw when sufficient melt has been accumulated in front of it,for injecting the melt into a mold through a valved injection nozzle.The machine was modified by augmenting the hydraulic system whichactuates the reciprocal movement of the screw so as to permit rapidinjection of the melt into the mold and by adjustment of the operatingmechanism to retain the injection nozzle in the injection positionthroughout the molding cycle and to provide for closure of the nozzle bymeans of the nozzle outlet valve as soon as injection of a charge offoamable composition had taken place. Additionally, the nozzle wasprovided with an electric heater and the sprue bush, which formed anintegral pillar portion of the fixed mold member, was provided with aconcave depression on the surface thereof which faced the injectionnozzle. This depression, which housed a thermally insulating materialformed therein as indicated below, was co-axial with the sprue channel,at its deepest point its depth was about 1/4 the thickness of the spruebush. The diameter of the depression was somewhat greater than that ofthe nozzle.

Cellular bowls were molded from high density polyethylene of melt index4.5 and density 0.960 g/cc, containing 1.5 percent by weight of an azodicarbonamide foaming agent, together with 2 percent by weight offinely-divided calcium silicate as foam nucleating agent, the latterbeing an optional component. Before the first molding was made, a discof a moldable insulating composition, about one-fourth inch thick, wasplaced between the nozzle and the sprue bush with the result thatforward movement of the nozzle into the injection position formed thedisc into the depression in the sprue bush, leaving the peripheralportion of the disc as an annular flanged portion bearing against thesurface of the sprue bush which surrounded the depression. injection ofthe first charge of foamable composition formed a central hole in theinsulation continuous with the sprue channel through which the foamablecomposition flowed without difficulty. The insulation remained in placeand unbroken during the injection molding run in which bowls wereproduced on a manufacturing scale. The mold used was made from castaluminum and was maintained at a temperature of about 50C during therun. After each injection of foamable composition, a cooling period ofseconds was provided before the mold was opened and the molded bowl wasejected. The main machine settings employed during the run were asfollows:

Cylinder temperatures rear l80"C middle 220C front 260C lnjection nozzle26$"C temperature Mold temperature 50C Cooling period 80 seconds By wayof comparison, bowls were molded as described above but without theinsulation. Under these circumstances, the base of the ejected bowl wasstill very hot in the region of the sprue due to heat transfer from theinjection nozzle, and this caused post-expansion" to take place in thebase of the bowl after ejection from the mold. This post expansioncaused the bowl surface to rise in a bubble which was both unsightly anda source of weakness due to the larger cell size thereof; additionally,of course, it formed an irregularity in the otherwise flat bottomsurface of the bowl so that the latter rocked about when placed on aflat surface. in a further comparative operation in which the injectionnozzle was retracted in conventional manner after injection of the melt,the material in the sprue channel foamed rearwardly towards the nozzleand united with material at the nozzle tip, thereby preventing ejectionof the molded article; this problem arose whether the insulation waspresent or not.

i claim as my invention:

1. in an injection molding assembly which is suitable for use in themanufacture of cellular plastics particles from a melt of a foamablethermoplastic composition and which comprises an injection moldingmachine having an injection nozzle and a mold assembly adapted toreceive a charge through said nozzle, the

improvement comprising the combination of an injection nozzle and a moldassembly wherein the injection nozzle is adapted to remain in itsinjection position with respect to the sprue bush of the mold assemblyduring the whole of the molding cycle and is insulated from the spruebush by a thermally insulating element between the outlet end of thenozzle and the sprue bush, the insulating element being apertured forthe passage of foamable melt therethrough and being disposed within arecessed portion of the sprue bush in combination with a correspondinglyapertured metal retaining element between said insulating material andthe outlet end of said nozzle, the outer surface of said retainingelement being recessed for the reception of said outlet end of thenozzle.

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